Powder metallurgy and ingot metallurgy are two commonly used processes to produce γ-TiAl sheets as illustrated in the flowcharts of FIGS. 1a and 1b respectively.
For the powder metallurgy process shown in FIG. 1a, expensive argon gas atomized powders are used as the starting material. The powders are canned in a titanium can, evacuated at elevated temperatures, sealed, and then hot isostatically pressed to a billet at 1,300° C. (2372° F.) for 2 hours in order to obtain complete densification. The billet is decanned and given a surface conditioning treatment. The cleaned billet is then encapsulated and isothermally rolled in the (α+γ) phase field to yield the desired thickness. The sheets are usually bent following rolling and are flattened at 1,000° C. (1832° F.) for 2 hours in vacuum. The canned material is then removed and the flat sheet is ground from both surfaces in order to achieve the desired thickness. The yield is high but the powder metallurgy produced sheet suffers from developing thermally induced porosity due to argon gas, which is entrapped in powder particles, and this limits its superplastic forming capability.
For ingot metallurgy process shown in FIG. 1b, the starting material is an as-cast γ-TiAl ingot. These ingots are subjected to hot isostatically pressing to close the shrinkage porosity commonly associated with cast ingots as well as to homogenize. These ingots are then cut into desired sizes and isothermally forged at 1,200° C. (2192° F.) to pancakes. Forging can be achieved either by single or multiple operations depending on the size of the ingots. Rectangular sizes are sliced from the pancakes by an electrical discharge machining technique and the machined surfaces are ground to remove the recast layer as well as to remove the forged surfaces prior to canning for isothermal rolling as described above. The yield is low for the ingot metallurgy process where a significant part of the pancake cannot be utilized. However, the ingot metallurgy produced sheets are amenable to superplastic forming, as they do not suffer from thermally induced porosity.
Consequently, there exists a need for a process for forming sheets of γ-TiAl alloys.